According to a conventional brake control apparatus, for example as disclosed in Japanese Patent Publication No. H06-107156, a spin of a vehicle on a μ-split road is suppressed by switching from an “independence limiting control” to a “select-low control” (or vice versa) for a front wheel on a high μ-road to carry out a braking operation, so that a braking force is obtained as much as possible.
The “μ-split road” here means a road, which has different coefficients of friction for a left wheel and a right wheel. The “high μ-road” means a road, which has a higher coefficient of friction.
According to a general ABS control, a control mode (a pressure decreasing mode, a pressure holding mode, and a pressure increasing mode) for each of wheels is individually selected depending on a slip ratio of respective wheels. According to the “independence limiting control”, either one of the left and right wheels (for which the slip ratio is larger than the other) is selected as a subject wheel for the ABS control. When the control mode for the subject wheel is in the pressure increasing mode, a control mode for the other wheel (for which the slip ratio is smaller) is read in. When the control mode for the subject wheel is in the pressure decreasing mode, and the control mode for the other wheel is in the pressure increasing mode, an increase inclination of a braking force is limited. According to such a control, a difference of the wheel cylinder pressure between the subject wheel and the other (non-subject) wheel is prevented from becoming too large.
The “select-low control” is a control, according to which a pressure decreasing operation is carried out for a wheel (right or left) on a high μ-road together with a pressure decreasing operation of ABS control for another wheel (left or right) on a low μ-road, irrespectively whether a condition for starting the ABS control is satisfied or not for the wheel on the high μ-road.
According to the independence limiting control, a larger braking force is generated for the wheel on the high μ-road than a braking force for the wheel on the low μ-road, so that the larger braking force is applied to a vehicle as much as possible. However, on a large μ-split road, on which the coefficients of friction between the left and right sides are largely different from each other, a difference of slip ratio between the left and right wheels would become larger. It would become more difficult to suppress a yaw torque applied to the vehicle and a vehicle spin may be likely to occur. Therefore, in the case of the larger μ-split road, not the independence limiting control but the select-low control is selected. Although, the braking force obtained in the select-low control is smaller than the braking force of the independence limiting control, the possible vehicle spin can be prevented by making the braking forces generated at the front left and front right wheels equal to each other, to thereby suppress the yaw torque applied to the vehicle.
According to another conventional brake control apparatus, for example as disclosed in Japanese Patent Publication No. 2001-260838, the EBD control is carried out for improving a braking efficiency, so that the wheel cylinder pressure for the rear wheels is controlled to become lower than the wheel cylinder pressure for the front wheels, to an extent that the wheel cylinder pressure for the rear wheels may not be largely departed from a brake fluid pressure generated at a master cylinder (a M/C pressure). Electromagnetic valves provided for respective wheels are controlled to be turned on and off (a fluid passage is opened and closed), so that the wheel cylinder pressure for a subject wheel is increased in a pulse form.
When pressure increase valves are simply opened and closed during the pressure increasing control for the ABS operation or for the EBD operation, the differential pressure may be generated more than necessary. As a result, noise and vibration may be generated. A linear control is carried out for the pressure increase valve, according to which a driving current to the pressure increase valve is controlled such that the differential pressure between an upstream and a downstream side of the pressure increase valve is linearly changed, in order to prevent the generation of the differential pressure more than necessary. As a result, the generation of the noise and vibration are suppressed.
However, when the linear control is carried out for the pressure increase valve, deviation of differential pressure with respect to the driving current may be generated due to individual difference of the pressure increase valves.
In the case that the pressure increase valves are provided for the left and right wheels, wherein the pressure increase valves have the deviation in their performances, a differential pressure of the wheel cylinder pressure generated at respective wheel cylinders for the left and right wheels may correspondingly have a deviation from a desired value. As a result, a difference of the wheel cylinder pressure between the left and right wheels may become larger, and the differential pressure for the wheel cylinder pressure of the respective left and right wheels may not be controlled within a predetermined range, even when the select-low control is carried out on the μ-split road. In such a case, the yaw torque may be generated in the vehicle. In particular, when the vehicle is running on the μ-split road, it may become difficult to sufficiently suppress the yaw torque to be applied to the vehicle and thereby it may become difficult to effectively prevent the possible vehicle spin.
On the other hand, if the difference of the wheel cylinder pressure between the left and right wheels would become smaller, the wheel cylinder pressure of the wheel on the high μ-road could not be made sufficiently higher relative to the wheel cylinder pressure of the wheel on the low μ-road, in spite that a larger braking force is desired to be generated for the wheel on the high μ-road. Therefore, in such a case, it may be a problem that a desired braking force may not be obtained as a whole.
In addition, when the EBD control is carried out, the driving current is likewise supplied to a solenoid of the pressure increase valve in order to generate a desired differential pressure at the valve. However, the desired differential pressure may not be generated, when the differential pressure actually generated is deviated from an expected value due to the individual difference, even though the driving current corresponding to the desired differential pressure is supplied to the pressure increase valve.
Furthermore, it may not be possible to increase the wheel cylinder pressure for the rear wheels by an amount of the deviation, when the wheel cylinder pressure for the rear wheels is controlled to gradually become closer to the wheel cylinder for the front wheels, if there exists deviation due to the individual difference of the pressure increase valve. Then, it would become difficult to obtain a desired braking force. In addition, it would be difficult to make the wheel cylinder pressure for the rear wheels equal to the wheel cylinder pressure for the front wheels.
As above, it may be a problem, if it would become difficult to control the braking forces for the front and rear wheels by properly distributing the braking force to the front and rear wheels during the EBD control.
The problems are explained above for the cases of the ABS control having the select-low control on the μ-split road and the EBD control. However, such problems (which would be caused by the individual difference of the pressure increase valve) may exist in any kinds of the brake control apparatuses, in which the differential pressure is generated by linearly operating the pressure increase valves so as to control the wheel cylinder pressure.